There are three stable isotopes of magnesium. Their masses are 23.9850, 24.9858, and 25.9826 amu.?

if Mg-24 has an abundance of 78.99%
then
100 - 78.99 = 21.01% is the abundance of (Mg-25 + Mg-26)

assuming that Mg-25 is X,..... then Mg-26 is (21.01% -x)


24.3050 amu = 78.99% of (23.9850) & X% of (24.9858) & ((21.01% -x) (25.9826)

24.3050 = (0.7899) (23.9850) & 24.9858X & (0.2101) (25.9826) - 25.9826X

24.3050 = (18.9458 & 24.9858X & 5.4589 - 25.9826X

0.9968X = 0.0997

X = 0.1000
Mg-25 = X = 10.00%

Mg-26 = 21.01% - X = 11.01%


your answers are
1) ^25Mg is 10.00%
2)^26Mg is 11.01%


answers match Lawrence Berkeley National Laboratory @ http://ie.lbl.gov/education/parent/Mg_iso.htm

Isotopes Of Magnesium